I have a question when specifying pressure drop across a control. If the upstream pressure is 300psig, and the fluid will flow to a downstream vessel operating at 30psig, would the delta P specified be 270psig? The fluid also has to travel about 200ft before reaching the downstream vessel, so how to take into account the pressure drop through the pipe?

This depends on what you are controlling.  If you need to control pressure AND you want the pressure at the vessel to be maintained at 30 psig, then the DP in the piping needs to be taken into account, or you could put your pressure sensing element in the vessel itself and use this as your control input.  However, if you need to control flow, then the valve will NOT control pressure.  You still need to take into account the DP in the piping, but you can specify the DP across the valve based on other flow criteria.

I fully support steamdog. To add a comment you would normally specify the delta P of the valve as the delta P critical for dimensioning the valve.

Put another way, what you need to know is following:
P1 immidiately before the valve
P2 immidiately after the valve
The flow through the valve.
Delta P definition: P1-P2
(In addition necessary pressure class taking into consideration pipeline pressure including pressure peaks)

If any of this is varying you would have at least have to add values for minimum, maximum and normal flow, and check the values for the proposed valve.

Reason:

Low delta P- high flow:
Pressure loss in pipeline before or after the valve could lead to a lower delta P at a high flowrate, and you could possibly have to increase the valvesize (Cv- value)
above what you would do in the normal situation, to get the wanted high flow through the valve.

High delta P-low flow:
Low flow figures at a minimum required flow could lead to low fricture loss before and/or after the valve and give a higher delta P than at 'normal' conditions.

You could possibly have to take into account cavitation problems that would not be critical at a more open valve with higher flow.

It could be necessary to select a valve(gate valve) with special trim or other construction to regulate better at low flow or to minimize cavitation problems.  

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